The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Emissions control is one factor in engine design and engine control. One particular emission, NOx, is a known by-product of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and water molecules in exemplary devices known in the art within the broader category of aftertreatment devices. Aftertreatment devices are known, for instance, utilizing chemical reactions to treat an exhaust gas flow. One exemplary device includes a selective catalytic reduction (SCR) device. An SCR utilizes a reductant capable of reacting with NOx to treat the NOx. One exemplary reductant is ammonia derived from urea injection. A number of alternative reductants are known in the art. Ammonia stored on a catalyst bed within the SCR catalysts reacts with NOx, preferably an equimolar mixture of NO and NO2, and produces favorable reactions to treat the NOx. It is known to operate a diesel oxidation catalyst (DOC) containing Platinum Group Metals (PGM, typically Pt and Pd) upstream of the SCR in diesel applications to convert a fraction of the NO into NO2 preferable to treatment in the SCR and additionally oxidize HC (hydrocarbons) and CO (carbon monoxide). A diesel particulate filter (DPF) device can additionally be used to trap and/or remove particulate matter from the exhaust gas flow.
NOx reduction efficiency in an aftertreatment system is temperature dependent. Testing shows in exemplary engine and aftertreatment configurations that up to 50% of total NOx emissions from a tailpipe of the configurations can occur during the vehicle cold-start before the downstream SCR catalyst is sufficiently active to react the NOx. NOx can be stored until it can be reduced selectively to nitrogen via reaction with urea/ammonia at higher temperature (for example, greater than 180° C.). However, an exemplary silver/alumina catalyst used for NOx storage is active when temperature is low (for example, greater than 75° C.)